video: lcd: Make splash_screen_prepare weak, remove config macro

Remove CONFIG_SPLASH_SCREEN_PREPARE from README
Add doc/README.splashprepare to document functionality

Signed-off-by: Robert Winkler <robert.winkler@boundarydevices.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
diff --git a/doc/README.splashprepare b/doc/README.splashprepare
new file mode 100644
index 0000000..61b4ec5
--- /dev/null
+++ b/doc/README.splashprepare
@@ -0,0 +1,8 @@
+---------------------------------------------------------------------
+Splash Screen
+---------------------------------------------------------------------
+The splash_screen_prepare() function is a weak function defined in
+common/splash.c. It is called as part of the splash screen display
+sequence. It gives the board an opportunity to prepare the splash
+image data before it is processed and sent to the frame buffer by
+U-Boot.  Define your own version to use this feature.